Covalent attachment of the hydrophilic polymer poly(ethylene glycol), abbreviated (PEG), also known as poly(ethylene oxide), abbreviated (PEO), to molecules and surfaces has important applications in biotechnology and medicine. In its most common form, PEG is a linear polymer having hydroxyl groups at each terminus:HO—CH2—CH2O(CH2CH2O)nCH2CH2—OHThis formula can be represented in brief as HO—PEG—OH, where it is understood that —PEG— represents the polymer backbone without the terminal groups: —PEG- equals —CH2CH2O(CH2CH2O)nCH2CH2—PEG is commonly used as methoxy-PEG—OH, or mPEG in brief, in which one terminus is the relatively inert methoxy group, while the other terminus is a hydroxyl group that is subject to ready chemical modification.CH3O—(CH2CH2O)n—CH2CH2—OH
It is understood by those skilled in the art that the term poly(ethylene glycol) or PEG represents or includes all the above forms and still others.
The copolymers of ethylene oxide and propylene oxide are closely related to PEG in their chemistry, and they can be substituted for PEG in many of its applications.HO—CH2CHRO(CH2CHRO)nCH2CHR—OH R=H and CH3
PEG is a useful polymer having the property of water solubility as well as solubility in many organic solvents. PEG is also non-toxic and non-immunogenic. When PEG is chemically attached to a water insoluble compound, the resulting conjugate generally is water soluble as well as soluble in many organic solvents. When the molecule to which PEG is attached is biologically active, such as a drug, this activity is commonly retained after attachment of PEG and the conjugate may display altered pharmacokinetics. For example, Bentley et al. in Polymer Preprints, 38(1), 584 (1997) demonstrated that the water insoluble antemisinin becomes water soluble and exhibits increased antimalarial activity when coupled to PEG. Davis et al., in U.S. Pat. No. 4,179,337 have shown that proteins coupled to PEG have enhanced blood circulation lifetime because of reduced kidney clearance and reduced immunogenicity. The lack of toxicity of PEG and its rapid clearance from the body are advantageous for pharmaceutical applications.
As applications of PEG chemistry have become more sophisticated, there has been an increasing need for heterobifunctional PEGs, that is PEGs bearing dissimilar terminal groups:
 X—PEG—Y
where X and Y are different groups. PEGs having backbone ester groups and terminal groups, X and Y:X—PEG—CO2—PEG—Ycan be considered to be heterobifunctional even if X and Y are the same, since each PEG unit within the backbone is substituted unsymmetrically.
Such heterobifunctional PEGs bearing appropriate functional groups may be used to link the PEGs to surfaces or other polymers, such as polysaccharides or proteins, with the other terminus attached, for example, to a drug, a liposome, another protein, or a biosensor. If one terminus is bound to a polymer, and the other terminus is bonded to an appropriate functional group, cross-linking to form a useful hydrogel can occur.
Utilizing existing methods, however, heterobifunctional PEGs are often difficult or impossible to prepare in high purity. For example, one could conduct the below reaction, using molar equivalents of each reagent with the goal of preparing the heterobifunctional PEG acetal product shown:HO—PEG—OH+ClCH2CH(OC2H5)2+NaOH→→HO—PEG—OCH2CH(OC2H5)2+NaCl+H2OIn practice, however, some of the disubstituted PEG diethyl acetal, (C2H5O)2CH2O—PEG—OCH2CH(OC2H5)2 is also inevitably formed and some unreacted PEG would also remain. Tedious chromatography would be required to separate this mixture.
The chromatographic approach has been used by Zalipsky (Bioconjugate Chemistry, 4: 296-299, 1993) to purify the following heterobifunctional PEG derivative:HO—PEG—CONHCH2CO2Hfrom a reaction product mixture also containing unreacted PEG and the disubstituted carboxylic acid derivative.
In certain applications, it is essential that minimum HO—PEG—OH be present in monoalkyl PEGs used to prepare monofunctional activated PEGs, since the presence of HO—PEG—OH would lead to doubly activated PEG derivatives which would result in crosslinked products or have other undesirable effects. In fact, HO—PEG—OH is a common contaminant in monoalkyl PEGs. The chromatographic approach has been disclosed in U.S. Pat. No. 5,298,410 to separate CH3O—PEG—OH from HO—PEG—OH by forming the trityl (Ph3C-derivatives), separating the derivatives chromatographically, and removing the trityl group from CH3O—PEG—OCPh3. A recent patent application, Suzawa, et al. (WO96/35451) disclosed benzyl PEG (C6H5—CH2—OPEG—OH) as an intermediate in preparing a heterobifunctional PEG bearing a group at one terminus having affinity for a target cell and having a toxin at the other terminus. The benzyl PEG, however, was prepared by benzylation of PEG, followed by laborious extensive gradient chromatography to separate benzyl PEG from dibenzyl PEG and unreacted PEG. The procedure was done on a small scale with a yield of only 7.8%. The method thus has little value for useful commercial production.
A second strategy, the polymerization approach, for preparing heterobifunctional PEGs involves anionic polymerization of ethylene oxide onto an anion, X−, which ultimately becomes the end-group of the polymer: This method has been used by Yokoyama, et al. (Bioconjugate Chemistry, 3: 275-276, 1992) to prepare a PEG with a hydroxyl group at one terminus and an amino group at the other. Cammas, et al. (Bioconjugate Chemistry, 6: 226-230, 1995) have used this method to prepare PEGs with an amino group on one terminus and a hydroxyl or methoxy group on the other. It has also been used by Nagasaki, et al. (Bioconjugate Chemistry, 6: 231-233, 1995) to prepare a PEG having a formyl group at one terminus and a hydroxyl group at the other. This method is generally useful only if X is a suitable and desired group on which to initiate polymerization; frequently this is not the case. Also, successful application of this method requires rigorous exclusion of water to prevent formation of HO—PEG—OH, and this problem becomes more severe as the molecular weight increases. It is also necessary to carefully control the degree of polymerization in order to obtain the desired molecular weight of the PEG derivative. This method is limited by the degradation of many types of drug molecules under the harsh conditions of the polymerization if the ethylene oxide polymerization is conducted directly on the drug molecule. The method is also limited by lack of selectivity if more than one functional group is present on which polymerization can occur.
It would be desirable to provide additional methods for preparing heterobifunctional PEGs that substantially eliminate at least some of the problems and drawbacks of previous methods.